1. Field of the Invention
The present invention pertains to a method and device for the identification of leakiness flaws in a neutron capturing (or absorbing) pencil (or rod) of a nuclear reactor.
2. Description of the Prior Art
In pressurized-water nuclear reactors used in nuclear power plants for the production of electrical power, the core of the nuclear reactor is formed by an assembly of enriched uranium rods between which neutron-capturing pencils are inserted in order to control the activity of the reactor. As a rule, as shown in FIG. 1a, the pencils are formed by hollow tubes made of stainless steel, the inside of each tube being filled with a metallic alloy formed by 80% silver, 50% indium and 5% cadmium. The tubes, in the case of a 900 MW power reactor for example, have a length of the order of 3.60 m. and are assembled in a cluster by means of a spider-support formed by a metallic part enabling the neutron-capturing pencils to be fastened by one of their ends. Thus, as shown schematically in FIG. 1b, in order to check the activity of the nuclear reactor, the neutron-capturing pencils, set up in a cluster, are introduced into the passage tubes made in the core of the reactor. The degree of penetration of the neutron-capturing pencils determines the instantaneous power of the nuclear reactor. The total penetration of the pencils corresponds to the shutdown of the reactor. During the working of the reactor, depending on the electrical power called up by the electrical power distributing network, the neutron-capturing pencils forming the cluster are, on the contrary, moved out of the passage tubes along about three-quarters of their length. The core of the reactor and the neutron-capturing pencils are then subjected to a very great flow of pressurized water, of the order of 3000 m3/s, in order to heat this water in contact with the core of the reactor. Owing to the flow of fluid to which they are subjected, the neutron-absorbing pencils should be held by guiding plates on their unoccupied part which is not engaged in the guiding tubes. The many swirls of pressurized water subject the neutron-capturing pencils, notably at the guiding plates and guiding tubes, to repeated shocks which, in the long run, cause a phenomenon of local wearing out of the corresponding pencils with the risk of loss of imperviousness of the steel tube forming each neutron-capturing pencil. The silver contained in the tubes forming the neutron-capturing pencils may become radioactive by neutron capture, notably at the lower part of these neutron-capturing pencils where the flow of neutrons is never negligible, and the risks of loss of imperviousness of the steel tubes forming each neutron-capturing pencil entails major risks of contamination of the environment and of people during the handling of the clusters outside the core of the reactor.
Furthermore, the risk of total breakage of the neutron-capturing pencils at their lower part is not negligible, and there is then the risk that these lower parts will sink to the bottom of the guidance tube. The effect of this, owing to the risk of jamming, is to create an additional risk of preventing the shutdown of the nuclear reactor. Systematic studies have shown that a rate of total breakage of 50% of at least of a neutron-capturing pencil is reached after a lifetime of four to five years.